It has previously been known that the level of a liquid can be determined using time domain reflectometry. In such time domain reflectometry systems, electrical pulses are conveyed along a transmission line to an electrically conductive probe extending down or otherwise through the vapor-liquid interface at the liquid level. The electrical pulses are partially reflected at the liquid level due to a change in the electrical impedance of the probe caused by the difference in the dielectric strength or the corresponding complex electrical permittivity between a gas or vapor existing above the liquid and the liquid itself.
U.S. Pat. No. 3,424,002 to Johnson shows systems which are stated as useful in determining the levels of liquids or of fluid solids. The Johnson patent discloses electrical step generators having relatively rapid rise times for generating an identifiable pulse or change. The step signal is transmitted to a coaxial probe having a center conductor and an outer cylindrical sheath which is grounded to the step generating system. The outer probe sheath is connected at the distal end of the probe to the center conductor thereof through a terminating resistor. The systems explained by Johnson also have timing subsystems for determining the amount of time between initiation of the stepped electrical signal and the receipt of reflected electrical signals generated at the impedance changes existing at the interface between the two different materials. Johnson recognizes that foams or emulsified interfaces cause a gradual increase in impedance, see column 1, lines 55-58.
U.S. Pat. No. 3,474,337 to Petrick teaches systems for sensing levels and electrical characteristics of fluent materials. The Petrick systems use the same general time domain reflectometry concept as Johnson. Petrick indicates that time domain reflectometry can be used to determine the dielectric constant and composition of a material having a fixed level.
U.S. Pat. No. 3,922,914 to Fuchs describes a bed level monitor designed for use in vessels containing dry products which are fluidized. The Fuchs systems are useful for sensing the level of fluidized or non-fluidized beds using a full length electrode which experiences reflected electrical signals at points of discontinuity such as the electrode terminal, the fluid bed level, and the end of the electrode. Fuchs mentions that gas bubbles cause irregularity in the time domain reflectometry (TDR) traces but fails to ascribe any particular significance or further usable information which can be extracted therefrom.
The prior art systems described above are concerned with recognizing interfaces occurring at relatively well defined liquid-gas, solid-gas, and liquid-liquid interfaces. Although such systems have been found useful for measuring such well defined interfaces they have not been found useful in systems such as boiling water nuclear reactors where there may not be a well defined interface at which the vapor and liquid phases meet. This is particularly important in the nuclear industry since high power output conditions can cause boiling or frothing conditions to occur over most, if not the entire depth of the reaction vessel. Under high output or melt-down conditions such as experienced at Three Mile Island, no such detectable level exists. Computerized control systems may malfunction if based upon prior art systems which require detection of a discernible portion of the reflected electrical signal pulse, such as caused by a definite change in impedance at a typical liquid-gas interface. Prior art time domain reflectometry liquid level detection systems are accordingly almost useless under such conditions and cannot be used to assure proper monitoring of the system over a wide range of boiling conditions.
Prior art TDR fluid level detection systems are also limited in that they have failed to provide an indication of total vessel liquid coolant inventory under boiling or frothing conditions. Such an inability to accurately monitor system coolant inventory increases the risk that loss of cooling water or other fluid will not be detected sufficiently early to prevent damage to the system. Such a limitation can be vitally important in nuclear reactors since key system operating parameters may not be capable of rapid adjustment in order to compensate for low cooling water inventory, particularly during times of high power output. A failure or catastrophe may occur as a result.
Prior art systems have also been limited by the requirement that the electrode or other probe extend downwardly from the terminal end in order to experience a relatively lower impedance to higher impedance interface. This arrangement may not be convenient in all applications. This prior art orientation has also been used to avoid the relatively greater attenuation which typically occurs in the denser material as compared to the overlying gas or other fluid.
Prior art time domain reflectometry fluid level detection systems have also implicitly required the influence of gravity or a similar force to concentrate the denser fluid or material for measurement. In space, such a gravitating influence typically will not exist for measurement of fluid systems. Fluid systems containing gaseous and liquid phases are commonly used in space vehicles. The phases are typically intermixed and non-homogeneously distributed within a vessel depending upon the various acceleration and gravitational forces acting thereon. The non-homogeneous distribution of the phases makes it difficult to accurately assess the mass contained in the vessel using prior art systems. Systems and methods using the present invention provide a means for determining fluid system inventory under such difficult conditions.
Prior art time domain reflectometry liquid level detection systems have also failed to provide methods or means for measuring turbulence of a fluid system. Turbulence can be an important process control variable in the nuclear industry, chemical process industries, and in other applications.